1. Field of the Invention
The present invention relates to a wiring structure using a flexible printed circuit board (hereinafter denoted as FPC board) and an optical module using the FPC board. In particular, the invention relates to a configuration of a FPC board with a stiff support and an optical module using the FPC board with the stiff support.
2. Related Prior Art
The FPC board is a freely bendable printed circuit board that provides a polymide base with a thickness of about 12.5 micron meters to 100 micron meters as a core material. Although it is bendable due to the thin core material, it tends to snap wiring patterns made of copper foil and formed on the surface thereof. Various support boards are generally used to prevent the wiring pattern from breaking. The support board can hold a portion of the FPC board in flat. However, when the FPC board is bent at the edge portion of the support board, the problem to break the wiring pattern may occur at such edge portion of the support board.
FIG. 4 shows how the wiring pattern on the FPC board is broken at the edge portion of the support board glued to the FPC board. FIG. 4A is a perspective view of the FPC board 101 with the support board 103, while FIG. 4B is a cross section taken along the line B-B shown in FIG. 4A. In figures, a member 101 is the FPC board made of, for example, polymide, a member 102 is a wiring pattern made of copper foil, and a member 103 is a support board made of, for example, glass-epoxy.
As shown in FIG. 4A and FIG. 4B, when the FPC board 101 provides the support board 103 in a side, the direction C in FIG. 4B, toward which the FPC board is to be bent by almost right angle, the edge of the support board 103, which is made of hard material, may damage or break the wiring pattern 102, though the support board 102 makes the FPC board 101 flat at the portion where the board 103 is stuck with.
The support board 103 to flatten a portion of the FPC board 101 may be often made of glass epoxy with a thickness from 0.3 mm to 1.0 mm, and stuck with the FPC board 101 with an adhesive. The wiring pattern 102 is assumed to foem in the side facing the support board. This configuration is often encountered in the optical subassembly (hereinafter denoted as OSA) connected with a circuit board by the FPC board 101.
FIG. 5 shows such configuration that the OSA 110 has a plurality of lead pins, 114 and 115, connected with the FPC board 111 by passing these lead pins, 114 and 115, into via-holes in the FPC board 111 and soldered to the lands 116 formed in the peripheral of the via-hole and extended from the wiring pattern 112. FIG. 6 is a side view of the OSA 110 with the FPC board shown in FIG. 5, in which the FPC board 111 is bent outwardly by about a right angle. FIG. 7 shows an example of the support board 120 to flatten the portion of the FPC board 111. The support board 120 has a plurality of via-holes, 121 and 122, positions of which correspond to respective positions of via-holes of the FPC board 111 and coincide with the arrangement of the lead pins, 114 and 115. One edge 120a of the support board 120 is a linear shape, while the other edge 120b fits to the outer shape of the OSA 110.
Assuming the arrangement that the wiring pattern 112 is formed on the surface not facing the OSA 110 in FIGS. 5 and 6. Bending the FPC board 111 outwardly to a direction D in FIG. 6, the lands 116 soldered with the lead pin 113 of the OSA 110 may peel off due to the stress caused by the bending of the FPC board 111, which results in the breakage of the wiring pattern. To prevent this failure, the support board 120 shown in FIG. 7 may be stuck to the FPC board 111 from the side E in FIG. 6. Thus, the support board 120 may prevent the lands 116 from peeling off from the FPC board 111. However, when bending the FPC board 111 at the edge portion 120a of the support board 120 with a small radius, typically smaller than 2 mm, the edge portion 120a may damage the wiring pattern 112 on the FPC board 111, which causes the breakage of the wiring pattern 112.
Various techniques are known to prevent the wiring pattern from the breakage by the bending of the FPC board. For example, Japanese Patent Application published as 2001-305568 has one method that a panel, which corresponds to the support board of the present invention, with a hollow at the bent portion of the FPC board, into which a swelling material is filled, promotes a slack bending of the FPC board to protect the breakage the wiring pattern. Another Japanese Patent published as H11-195855 has disclosed a deformation-protecting member made of stiff material, typically made of metal, provided inside the bent portion of the FPC board. Still another Japanese Patent published as H06-326425 has disclosed another method to protect the wiring pattern that the FPC board provides a protective film on the surface thereof and only the edge of this film is fixed to somewhere to prevent the wiring pattern from the damage and the breakage due to the external force.
However, these prior arts may not apply to the case that the FPC board stuck with the support board is bent with a relatively small radius, in particular, they may not apply to the optical subassembly with a small package and dense lead pins.
The present invention is to solve the above subject, namely, the present invention is to provide a wiring structure that, in the FPC board with the support board, when the FPC board is bent at the edge portion of the support board, the wiring pattern formed on the FPC may be not damaged or broken by the edge of the support board.